1. Field of the Invention
The object of the present invention is a fan intended in particular for equipping a tunnel for refrigeration of products such as food products, and also the tunnel equipped with such a set of fans.
2. Description of the Related Art
It is known that in cryogenic tunnels for refrigeration of food products a cold gaseous current is circulated which ensures heat transfer by convection between the cold gas and the food product to be refrigerated. The aim is to ensure high levels of heat transfer without damaging or moving the products on the belt of the conveyor, while minimising the energy dissipated inside the cryogenic tunnel.
This type of tunnel has been used for more than thirty years with a wide variety of fans and of devices to ensure the necessary gas movement. The most common fans act axially and are fitted so as to blow downwards in the direction of the food product. Centrifugal fans which push the gas transversely to the products are also used. The heat transfer coefficients of such systems normally lie between 45 and 90 W/m2/xc2x0 C. Tunnels optimised for treatment of products that are in principle thin such as small hamburger rounds use gaseous vortices whose heat transfer coefficient is about 120 W/m2/xc2x0 C. or which produce gaseous jets impinging on the products with a heat transfer coefficient greater than 200 W/m2/xc2x0 C.
The gaseous vortices normally require that the gas flows away through zones not covered by the belt of the conveyor, which limits their application to products which do not completely cover the belt of the conveyor, an upper limit of 65% for the coverage of the belt being usual.
The gaseous impact systems are normally based on fans which create an overpressure in a relatively large volume of the tunnel. The gaseous jet impacts are produced by a gas which is expelled from a high pressure volume through a plurality of holes, or of channels. So that these impact systems can be used in cryogenic tunnels, they must be easy to clean, and special characteristics are required to maintain their performance owing to the potential presence of snow and of ice formed in the holes and tubes or the channels.
In many cases, problems arise in connection with excessive transverse air currents or suction towards the fans which can cause movement of light products on the conveyor. For most products which could be moved, such movement is unacceptable as it can damage or modify the shape of the product or make it come out of alignment with the other products, even when that alignment is intended to allow a subsequent process such as automatic packing.
For products which are not particularly fine, the highest heat transfer coefficients do not contribute major advantages in terms of treatment speed, as the thermal diffusion capability of the products limits the rate of heat transfer from the products.
The object of the invention is therefore to propose a fan designed to remedy these disadvantages.
The fan concerned in the invention is intended in particular to equip a tunnel for refrigeration of products such as food products, this tunnel having a cold gaseous refrigerating current flowing through it.
According to the invention, this fan comprises at least one pair of funnels fixed onto one face of a support mounted so that it can rotate about a vertical axis and connected to a means of driving rotation, and these funnels have their entrance apertures arranged radially so as to allow the gaseous current to penetrate into them and to emerge from them at an accelerated speed via their exit aperture.
A set of these fans can be appropriately arranged inside the tunnel, and the funnels of each fan can be profiled to generate a gaseous jet moving for example at a speed of 20 to 30 m/sec and which impinges on the product to be refrigerated. The high speed of the gaseous jet emerging from the fans provides high levels of heat transfer as the boundary layer around the product is very thin. The action of the fan does not create high suction flows nor zones of very low pressure, which makes it possible advantageously to minimise any movement of the products.
The cryogenic tunnel also concerned in the invention is equipped with a set of these fans, arranged to direct cold gaseous currents locally at an accelerated speed onto predetermined zones of the conveyor.